Equipment and Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 305

in the threshold zone of the tunnel. The highest lighting levels to be installed in all
the following zones will depend on this threshold-zone level. Tools will be described
that are needed to facilitate the determination of this level in practice for tunnels that
are already in place as well as for tunnels still in the design or construction phase.
Guidelines on the arrangement of the luminaires, on lighting control in response
to varying daylight situations, and the all-important subject of maintenance will be
given in a further part of this section.

22.5.1 Tools for the Determination of L 20 and L seq

As has been discussed in detail in Sect. 20.2, different concepts have been devel-
oped to determine the required threshold-zone luminance level corresponding to the
brightest daylight situation occurring at least 75 h annually (75 h being recommended
by CIE).
The two most important concepts are the L 20 and L seq concepts. In the case of
existing tunnels, the values of L 20 or L seq can be measured on site with appropriate
measuring equipment. However, it is difficult, if not impossible, to measure at the
moment when the “75 h daylight situation” happens to occur. It is therefore more
realistic to determine the values of L 20 or L seq based on photographs or, in the case
of tunnels yet to be constructed, on perspective drawings of the tunnel as seen by
the approaching motorist from a safe stopping distance in front of the tunnel. The
luminance values of the different surfaces surrounding the tunnel entrance occurring
at the moment of the “75 h daylight situation” can, for example, be obtained from
Sect. 20.2.1 “Daylight Luminances in the Access Zone” (Table 20.1 ). This then forms
the basis for the determination of both L 20 and L seq .

22.5.1.1 Determination of L 20

A20 ⓦ circle is drawn on the tunnel photograph or drawing, scaled from the perspective
of the safe stopping distance. The known height of the tunnel entrance can be the
basis for sizing the 20 ⓦ circle. An example of the result is shown in Fig. 22.4 . From
the photograph or drawing the percentage (p) in the 20 ⓦ circle of the sky, of the road
surface and of the different types of surrounding surfaces is determined. L 20 follows
from the formula:

L 20 = p sky ∗ L sky + p road ∗ L road + p grass ∗ L grass + p mountain ∗ L mountain

The values of the sky, road, grass and mountain luminances can then be taken from
Table 20.1.